Class 9 Chemistry: Atoms and Molecules
Chemical formulae are compact descriptions of composition. Reading them well means noticing which elements are present and how many particles of each are represented.
What to understand
- Describe the central idea in your own words.
- Use a model, observation or relationship to explain a changed example.
- Notice and correct the likely misconception in the worked reasoning.
Read the formula
An atom is a small unit of an element; a molecule contains atoms joined in a definite arrangement. In a formula such as H2O, the subscript 2 belongs to hydrogen atoms, while the absence of a subscript means one oxygen atom.
A coefficient counts whole particles: 2H2O means two water molecules and therefore four hydrogen atoms and two oxygen atoms. Conservation of mass is easier to see when each element is counted on both sides of a reaction.
Try, explain and check
How many oxygen atoms are in 2Al2(SO4)3? Explain the grouping.
Show a reasoning prompt
Why may a coefficient change during balancing while a subscript should not be changed casually?
Show a reasoning prompt
Use the chemical reactions worksheet for a separate balancing practice set.
Show a reasoning prompt
Next step
Write a three-sentence revision summary, then continue to the related resource. For extra practice, visit the resource library and choose a worksheet that matches the skill you need.